In the process of production of IC devices, a test apparatus for performing power-up tests on IC devices is referred to as an “IC tester”. Further, a conveyor apparatus for conveying IC devices for power-up tests by an IC tester is referred to as an IC handler. An IC tester is designed to push IC devices attached to a test head through test sockets against the test head so as to run current through the IC devices. A device for pushing IC devices in the sockets in this way is called a “contact head”. In recent IC handlers, the contact head is attached to a robot arm operating so as to insert the IC devices to the sockets.
In this regard, if an IC device inserted in a test socket is not ejected from the socket for some reason or another and is left there, a newly inserted IC device will be stacked over the IC device remaining in the socket. Such a situation can occur if, for example, a worker inserts a dummy device for measurement of resistance in a socket for inspection of the test head, then forgets to eject the dummy device from the socket. If two IC devices are stacked in a socket, the IC device remaining in the socket continues to electrically contact the test head, so it is no longer possible to obtain accurate test results of the newly inserted IC device. Further, if the IC devices inserted stacked in the socket are pushed by the contact head, these IC devices or the contact head may also be damaged. Therefore, a technique suitably preventing a state where two IC devices are loaded stacked in a test socket has been considered necessary. Such a state will be referred to below as a “double stacked state” of IC devices 2.
In relation to this, PLT 1 discloses the art of placing a fiber sensor in a socket for firing a light beam cutting across the socket and judging if an IC device remains in the socket in accordance with whether the light beam of the fiber sensor is interrupted. Further, PLT 2 discloses the art of placing a line sensor or area sensor or other imaging device above a socket and analyzing image data of the socket obtained at the imaging device to judge if an IC device remains in the socket. More specifically, PLT 2 discloses to compare reference data prepared in advance for each type of socket and image data obtained in the imaging device to thereby judge if an IC device remains in the socket.
However, according to the simplified technique using a fiber sensor such as PLT 1, when the IC device under test is a thin type (for example, when the thickness of the IC device is 0.5 mm or less), sometimes it is not possible to accurately detect the IC device remaining in the socket. Furthermore, according to the technique of PLT 1, it is necessary to precisely position the light axis of the fiber sensor with respect to the socket each time the dimensions of the IC device are changed, so the load of the preparatory work by the worker was great. Further, according to PLT 2, each time the color, shape, etc. of the IC device or socket was changed, was necessary to adjust the position, amount, etc. of the light applied to the socket or produce new reference data, so the load of the preparatory work by the worker was again large.